It has long been known that the formation of an oxidized layer, generally in the form of magnetite, on the surface of the exchange tubes decreases the performance of the exchanger and, moreover, promotes inter-granular corrosion and the appearance of stress cracks. Consequently, it is common periodically to remove the magnetite by leaching the tubes. Various cleaning agent types are known, generally acting by reduction and then complexation of the magnetite. In particular, a cleaning process by reduction using N.sub.2 H.sub.4, followed by complexation of the Fe.sup.2+ to a compound of Fe.sup.3+ using ethylenediaminetetraacetic acid, has been used. The various well-known processes which exist will not be described in the present application.
Analyses carried out on samples of exchanger tubes bearing deposits have demonstrated that the structure of these deposits was much more complex than was imagined. They have in particular shown that, if the deposit consists mainly of magnetite, there may, locally, be silicates present on an alveolar silico-aluminate having a solid gel structure. The silicates may contain carbon-containing products insofar as this element is present in the medium. The oxidized passivation layer which is conventionally produced during the final phase of manufacture of the exchange tubes is destroyed under the alveolar silico-aluminate and is substituted there by a relatively thick, non-protective layer. The analysis showed that this layer has a high content of chromium and of iron in the usual case where the exchange tubes are formed from one of the nickel-chromium alloys also containing iron and other addition elements, referred to by the brand name "INCONEL". It is also liable to exist, with a lesser thickness, under the magnetite in the straight parts of the tubes.